The reproduction of continuous tone images is traditionally done by full contone reproduction such as colour photo prints or by binary halftoning techniques such as colour offset printing. Electrographic printing, where a latent image is formed by static electricity that is locally discharged to form graphical representations, has an important member called electrophotographic printing. In electrophotographic printing, the static energy is partially removed by a directed light beam. Electrophotographic printers have traditionally a capability of binary printing. The illusion of continuous tone images is reached by binary halftoning techniques. Every addressable spot on the output, further called micro dot, can get a high density corresponding with full toner coverage or a low density corresponding with the absence of toner.
Recently, electrographic printers have also limited contone capabilities. That means that the amount of toner per micro dot can be modulated more continuously, such that the micro dot--after rendering--can have apart from a low density and a high density also some mid densities. The density level can be regulated by an energy level that is applied to the micro dot by an output device. Agfa-Gevaert N.V. from Mortsel Belgium markets such an electrophotographic printer under the trade name Chromapress. This is a duplex colour printer (cyan, magenta, yellow, black) having a resolution of 600 micro dots per inch producing 1000 A3 pages per hour. Per micro dot, 64 energy levels can be selected. The output device can be also a thermographic printer, inkjet printer, more generally an electrographic printer etc. The problem with the mid densities is that these are not stable as a consequence of the physics of the electrographic process. By instability is meant that there is not a one to one relation between the energy level applied to the device and the density level obtained on the reproduction. The density level of a first micro dot is strongly dependent on the energy level applied to the micro dots in the direct neighbourhood of the first micro dot. Therefore several methods have been proposed to enhance the stability of the micro dots. This can be done up to a certain limit dependent on the density level. An important aspect of the remaining instability is that not enough density levels per micro dot can be rendered. Therefore a technique related to binary halftoning must be used, which is called multilevel halftoning. A problem with halftoning is that the spatial resolution is decreased to improve the density resolution. Another problem is that internal moire can show up due to the interaction between the micro dots and the halftoning pattern. These problems have been addressed in WO-A-93 26116, for multilevel halftoning of images having one colour component. FIG. 7 of that application discloses a 3-bit grey halftone dot layout according to a mixed dot type embodiment. For low output densities, isolated halftone dots appear on a background. The halftone dots comprise microdots having two different density levels. For higher output density levels, isolated bands appear and for the highest densities, maximum two different density levels are present in each halftone cell.
However, if different colour components are printed on top of each other to get colour reproductions, colour moire can occur between the different components. This problem is not addressed in the above mentioned application. Colour moire or inter image moire is different from internal moire, as will be described below.
EP-A-0 370 271 discloses the formation of halftone dots to prevent rosette moire and colour shift, but is related to binary halftoning. Problems of stability of micro dots and aliasing of line structures by use of multiple levels are not addressed in this application.